On this day in 1999, Landsat 7 was launched into orbit. Landsat 7 is the considered the most accurately calibrated Earth-observing satellite, i.e., its measurements are extremely accurate when compared to the same measurements made on the ground. The excellent data quality, consistent global archiving scheme, and now free data of Landsat 7 are hallmarks of its impressive tenure.

The Landsat 7 mission went flawlessly until May 2003 when a hardware component failure left wedge-shaped spaces of missing data on either side of its images. Despite, these problems and the resulting data loss of 22 percent of each image, Landsat 7 data is still highly useful for many scientific applications.
From USGS:
On Tax Day, April 15, taxpayers will see another wise return on their investment! The satellite Landsat 7, which is managed by the USGS, will complete 10 years of recording features of the land mass of the planet. The satellite was designed to last only 3 to 5 years but continues to provide important information about the land surface of the planet. Landsat 7’s companion satellite, Landsat 5, recently achieved 25 years of operation, which surpassed its designed life expectancy by 22 years. Virtually every major alteration of the land surface of the planet, from the effects of wildfires, tsunamis, hurricanes, to urban growth has been recorded by one of the Landsat satellites and the data are used by scientists, disaster management teams, and government agencies. All Landsat data are available at no cost through a USGS online system, USGS Global Visualization Viewer.